In many liquid crystal display (LCD) configurations, and particularly those employing the commonly-used twisted nematic (TN) phase, the brightness of a pixel is modulated by the voltage applied across the liquid crystal (LC) cell. The voltage affects the degree to which the LC material rotates polarized light, which in turn controls how much light passes through an exit polarizer. In other words, a LCD is a passive device that acts as a light valve. The managing and controlling of data to be displayed is typically performed by one or more circuits, which are commonly referred to as display driver circuits or simply drivers.
Grayscale can be achieved by driving varying analog voltages to LCD pixels. Analog video amplifiers are often used in the video signal path of LCD driven circuits. If the video signal source is digital, then one or more digital-to-analog converters (DACs) will typically be used to convert the digital video signal into a corresponding analog video signal. An important consideration in the design of video electronics is the power dissipation of these analog circuits because the DACs and amplifiers can account for a significant, or even dominant, portion of the system power budget.
Some display applications require pixels driven to purely white or black, and do not use intermediate gray levels. Such purely white or black applications are referred to as bi-level video systems. With only one bit per pixel, these bi-level video systems can often be simpler to drive than grayscale systems, since the DAC and video amplifier and can often be replaced with a switch to select between the voltages associated with driving a LCD to black and white.
Generally, LCDs do not work well with direct current (DC) voltages. A graph of transmission versus voltage applied to a LCD is shown in FIG. 1. High transmission occurs with zero voltage and low transmission with either positive or negative voltage. Therefore, to drive a LCD to black, a positive or negative voltage can be applied to the LCD. However, driving a LCD at a steady state DC voltage may damage the display by, for example, causing contaminants to plate on one side or the other of the LC cell. In order to prevent damage, the voltage applied to the LCD is generally flipped back and forth (alternated) between high-black and low-black, to preserve zero (0) DC voltage, also called DC restore.
There are different scenarios for preserving zero volts DC (0 Vdc), as shown in the series of succeeding frames of FIGS. 2A-2D. One scenario uses column inversion as shown in FIG. 2A, where one frame is written with all the columns having alternating polarity, positive-negative, and positive-negative. In the next frame all the columns are written negative-positive, negative-positive. In the succeeding frame, all the columns are again written positive-negative, positive-negative. As shown in FIG. 2B, frame inversion can be used where the first frame is written with all positives and the next frame is written with all negatives. The succeeding frame is again written with all positives. As shown in FIG. 2C, pixel inversion can be used which produces a checkerboard like effect in the first frame and an inverted effect in the second frame. In the third frame, the checkerboard like effect matches that of the first frame. Lastly, as shown in FIG. 2D, row inversion can be used where all the rows are alternating polarity, positive-negative, and positive-negative. In the next frame all the rows are written negative-positive, negative-positive. In the third frame, the rows are again written positive-negative, positive-negative.
One approach to implementing an alternating current-coupled (AC-coupled) display driver circuit with one or more direct current-restore (DC-restore) switches integrated within a LCD is U.S. Pat. No. 7,138,993, by Frederick P. Herrmann, issued on Nov. 21, 2006, and assigned to Kopin Corporation of Taunton, Mass., the entire contents of which are hereby incorporated by reference.